After import to Unity, back faces are transparent
There are a few different things that might be causing your problem.

Firstly, there's no way to add thickness without increasing the tri/poly count. Polygons only have one face, so if you view them from the "wrong" side, they are invisible. There is no way to avoid this. If you want to see both sides, you have to have another polygon that is facing the other direction.

The first step is to make sure that when you add thickness in Blender, that you're adding REAL thickness, instead of using a modifier. If you do use a modifier, you need to "apply" before you export to Unity (there's a way to apply on when you export, but I won't talk about that right now). Either way, they have to be "real" polygons that get exported, not just the result of a modifier.

The next step is to make sure that all of the normals of your polygons are facing the right way (outward). If you select all of your polys in Blender and press ctrln then it should recalculate all of your normals "outside."

Once you've done these two things, try exporting to Unity again, and see if that helps.